As a power supply for driving mobile devices such as a cellular phone and laptop computer, a lithium-ion secondary battery having high energy density and high capacity has been widely used for a long time. Further, lithium-ion capacitors having several times as much energy density as conventional EDLC (electric double-layer capacitor) are actively developed. In addition, the carbonaceous material is used as the negative electrode material, and particularly, the graphitic carbon material (graphite) is widely used.
Recently, small-sized lithium-ion batteries and lithium-ion capacitors capable of storing high energy are developed as a clean energy source for electric cars such as electric vehicle (EV) and hybrid electric vehicle (HEV), combining an internal combustion engine and batteries, and thus, they are expected to be served as a next generation secondary battery for vehicles. However, use conditions of the secondary battery for vehicles are severe compared to those of consumer applications. That is, it is required to improve charge-discharge capacity and cycle life in addition to the requirement of high energy density. In particular, high input/output performances in room temperature, and rapid output performances under low temperature for engines starting in cold areas, i.e. low-value resistance under low temperature is also required. In connection with this, the graphitic carbon material has an excellent energy density, but input/output performances thereof are poor due to high crystallinity. In particular, a problem such as a risk of lithium metal deposition under low temperature has been pointed out. On the other hand, a non-graphitizable carbon material (hard carbon) has excellent input/output performances under low temperature. Further, it has a certain characteristic in that the risk of lithium metal deposition under overcharge is low. Even further, it has an excellent characteristic in that SOC (State of charge) is easily controlled due to a character that an electric potential changes depending on the amount of doped lithium. Therefore, the non-graphitizable carbon material is developed as a negative electrode of the next generation secondary battery for vehicles.
For example, JP PATENT No. 4023504 (Patent literature 1) proposes a negative electrode material for non-aqueous electrolyte secondary batteries containing a carbonaceous material derived from plant-originated polymer, which contains a total of 0.2 to 20 weight % of metallic elements of Na, K, Ca, Mg, Al, and Si, and phosphorus, and sulfur, as total elements.
In Patent literature 1, the metallic elements are involved in the carbonaceous material derived from plant-originated polymer, to thereby reduce a crystallinity of the carbonaceous material. As a result, the carbonaceous material becomes a non-graphitizable carbon material. However, the carbonaceous material contains a large amount of metallic elements. Therefore, the carbonaceous material has adverse effects on battery performance, and further, the crystallinity and a pore-uniformity thereof are also low.
Japanese Unexamined Patent Publication No. 6-89721 (JP Patent No. 3399015; Patent literature 2) discloses a non-graphitizable carbon material obtained by heat treating at 600° C. under an atmosphere condition wherein volatile-matters developed from a carbon precursor are removed from the reaction system. The non-graphitizable carbon material is a negative electrode material having a ratio by weight (Ps) of less than 0.59 which is one of a stacking structured-carbon atom, or a stacking index (SI) of less than 0.76. Further, Japanese Unexamined Patent Publication No. 9-161801 (JP Patent No. 3719790; Patent literature 3) discloses a carbonaceous material for an electrode obtained by carbonizing a plant-derived organic substance selected from the group consisting of palm shell, chaff, broad-leaf tree, needle-leaf tree, and bamboo. The carbonaceous material is for an electrode of non-aqueous, electrolyte secondary batteries and has 1 mL/g or more of a volume of pores having a diameter of 0.003 to 5 μm, 100 m2/g or less of a specific surface area determined by a BET method, and 5 to 100 μm of an average particle diameter. However, these carbonaceous materials are heat treated under vacuum, and thus pores are excessively developed in sites wherein volatile-matters are actively removed. Therefore, these carbonaceous materials provide causes of increase of irreversible capacity and decrease of cycle performances.
Therefore, carbonaceous materials for a negative electrode of non-graphitizable carbon material wherein an entrance diameter of the pores is controlled, are examined. In particular, Japanese Unexamined Patent Publication No. 7-230803 (JP Patent No. 2844302; Patent literature 4) discloses a carbonaceous negative electrode material for lithium secondary batteries wherein the entrance diameter of the pores on the surface of the non-graphitizable carbon fine particles which constitutes carbonaceous negative electrode material, is controlled, so that lithium ions in the electrolytic solution of lithium secondary batteries can pass the entrance of the pores and organic solvent therein cannot substantially pass the entrance of the pores. In this Patent literature, the adsorbability of organic solvent in the non-graphitizable carbon material is reduced and the discharge capacity of the non-graphitizable carbon material is improved by depositing pyrolyzed carbon in the surface of non-graphitizable carbon. Further, in the working examples, the non-graphitizable carbon obtained by heat treating palm shell chars after dealkalization treatment using hydrochloric acid, is flowed together with nitrogen gas saturated with toluene while heating, to thereby deposit pyrolyzed carbon in the surface. An irreversible capacity of the carbonaceous material for a negative electrode is reduced. However, an internal resistance based on the carbon material per se is increased at a low temperature, and thus a sufficient output performance for a short time cannot be obtained.
Japanese Unexamined Patent Publication No. 2007-42571 (Patent literature 5) proposes a carbon particle for a negative electrode of lithium secondary batteries wherein an average (002) interlayer spacing d002 determined by XRD measurement is 0.340 to 0.390 nm, a He true density is 1.40 to 2.00 g/cc, and an amount of CO2 adsorption is 0.01˜500 cc/g, as a negative electrode material having a low irreversible capacity and an excellent output performance. In Patent literature 5, carbon particles having little fine pores can be obtained by heat treating a resin constitutionally containing a specific phenol derivative, and thereby the irreversible capacity of a battery using the carbon material is reduced. However, the input/output performances of the carbon material at a low temperature are not sufficient. Further, the charge-discharge capacity is also reduced by pore decrease.
Further, WO2005/98999 (Patent literature 6) discloses a carbonaceous material obtained by heat treating petroleum or coal tar in a specific condition, wherein an average (002) interlayer spacing d002 is 0.355 to 0.400 nm and a true density is 1.50 to 1.60 g/cm3. The carbonaceous material which is a negative electrode material for non-aqueous electrolyte secondary batteries, has excellent input/output performances at high current. However, the carbonaceous material does not have sufficient input/output performances at low temperature. Further, a main component of the carbonaceous materials in Patent literatures 5 and 6 is a synthetic polymer prepared from fossil resource. Thus, a shift from the fossil resource to a biomass material has been demanded, from the viewpoint of a conservation of the global environment.
On the other hand, a discharge capacity per charge of the electric double-layer capacitor is low compared to the lithium-ion secondary battery, but the electric double-layer capacitor has excellent, instant input/output performances. Further, in the electric double-layer capacitor, charge and discharge can be performed over several tens of thousands of cycles, and thus the electric double-layer capacitor is maintenance-free. Recently, a hybrid capacitor which is a combination of the lithium-ion secondary battery with a principle of electricity storage in the electric double-layer capacitor, has been developed (Patent literature 9). In such an electric double-layer capacitor, lithium-ions are stored and doped in the negative electrode. The non-graphitizable carbon material is expected to serve as the negative electrode.
The inventors of the present invention found that the carbonaceous material for a negative electrode prepared by using the plant-derived organic substance as a carbon source can be doped with a large amount of active material, and thus it has promise as a negative electrode material (Patent literatures 3 and 7). However, when the plant-derived organic substance is used as a carbon source of carbonaceous material for a negative electrode, potassium found in organic, raw material had an unsuitable effect on the doping and dedoping performance. In order to solve the above problem, Patent literature 7 discloses a method for reducing an amount of potassium wherein the plant-derived organic substance is washed by acid so as to demineralize (hereinafter referred to as a demineralization in liquid-phase) (Patent literature 7). That is to say, in the method for preparing a carbonaceous material for a negative electrode wherein the plant-derived organic substance is used as a carbon source, the demineralizing treatment is required.